Autothrottle does not command a pitch-up or pitch-down moment. It commands a throttle up or throttle down instruction. Pitch up or pitch down is a direct consequence of a trim change on release of the autopilot. Usually exacerbated by improper trim when the a/p was previously engaged, or some variation introduced due to CG, fuel or pax change during flight.
Please provide evidence that the autothrottle commands a control surface movement.